Re: syntax taste: use of unquote in macros

[Ludovic Courtès]

Hi Matt,

Matt Wette <address@hidden> skribis:

> I'm not sure if you know about this, but there is a discrepancy in the
> way some folks define macros to use unquote (aka ,).   For example,
>
>> (use-modules (system base pmatch))
>> (pmatch '(foo "bar")  ((foo ,val)  (write val) (newline)))
> => "bar"
>
>> (use-modules (ice-9 match))
>> (match '(foo "bar")  (`(foo ,val)  (write val) (newline)))
> => "bar"
>
> Note the difference in the use of quasiquote (aka `) in the pattern
> for (foo ,val): match syntax uses it, pmatch does not.
> In Scheme, quasiquote and unquote always come together.
>
> Is pmatch syntax in bad taste?  I'm looking for opinions.

It really depends on what you’re going to use the pattern matcher for.
For ‘sxml-match’, it’s more convenient to have literals be the default
because there are usually more literals than variables, as in:

       (sxml-match x
         ((album (@ (title ,t)) (catalog (num ,n) (fmt ,f)) ...)
          `(ul (li ,t)
               (li (b ,n) (i ,f)) ...)))

In more general cases, I prefer the (ice-9 match) style because patterns
typically have more variables than literals.

In one case, I found myself implementing pmatch-style quoting on top of
(ice-9 match) so I would have the best of both worlds:

  https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/installer/tests.scm#n84

:-)

Ludo’.